The present invention relates to a fixing device used in an electrostatic recording type image forming apparatus such as a copying machine, a facsimile device, a printer, etc. and more particularly to a fixing device of a toner image using an electromagnetic inductive heating system, and an image forming apparatus using the fixing device.
In recent years, factors such as energy saving and high speed have been commercially increasingly requested for an image forming apparatus, for instance, a printer, a copying machine, a facsimile device or the like. To achieve these requested performances, it is important to improve a thermal efficiency of a fixing device employed for the image forming apparatus.
In the image forming apparatus, a non-fixed toner image is formed on a recording medium such as a sheet material, a printing sheet, a photosensitive sheet, an electrostatic recording sheet, etc. in accordance with an image transfer system or a direct system by an image forming process of electro-photographic recording, electrostatic recording, magnetic recording or the like. As fixing devices for fixing the non-fixed toner image, contact heating type fixing devices such as a heat roller system, a film heating system, an electromagnetic inductive heating system, etc. are widely employed.
As the electromagnetic inductive heating type fixing device, European Patent Publication EP-1174774 proposed that Joule heat is produced by eddy current caused in a heat generate member made of a magnetic metal member under the alternating magnetic field of inductive heating means made of a magnetizing coil. Thus, in this technique, the heat of the heat generating member is generated electro-magnetically and inductively.
Since the heat roller is made of a magnetic member, a magnetic path of a magnetic flux generated by energizing a magnetizing coil is formed. However, when a back surface core does not exist, the magnetic flux leaks outside. Accordingly, the back surface core is provided to form the magnetic path to prevent the magnetic flux from leaking outside.
When a plurality of C shaped cores are provided in the direction of the circumference of the heat roller as usual, magnetic flux density in the parts of the C shaped cores is high, however, magnetic flux density in the parts where the C shaped cores are not present is low. Therefore, the temperature of the heat roller in the parts where the C-shaped cores exist excessively rises relative to the temperature of the heat roller in the parts where the C-shaped cores are not present to generate an excessive fixing (hot offset) in the parts.
As compared therewith, since the temperature of the heat roller in the parts where the C-shaped cores are not present is relatively low, fixing characteristics are insufficient. Accordingly, such inconveniences as uneven fixing characteristics and uneven brightness arise in the parts where the C-shaped cores are present and the parts where the C-shaped cores are not present.